The present invention relates to a magnetic tape device having an improved data processing ability. More particularly, the present invention relates to a magnetic tape device which selectively executes data access requests from a host apparatus in accordance with predetermined rules which reduce the need to reposition the tape between data access requests. The present invention further relates to a magnetic tape device capable of storing individual data items into plural non-continuous regions on a tape medium in order to facilitate the allocation of additional space for individual data items, without sacrificing storage efficiency.
In a conventional magnetic tape apparatus such as the apparatus shown in FIG. 1, a command issued from a host apparatus 10 to a magnetic tape apparatus 20 is received by a driver/receiver 21. The driver/receiver notifies a controller 23 regarding the reception of a command and stores the command and related parameter(s) to a buffer 22.
Upon initial mounting of a tape, the control apparatus reads volume management information 25 from the magnetic tape medium and stores the information to a memory 24. The volume management information 25 stored in memory 24 is updated each time a write process is conducted to the magnetic tape apparatus, and is subsequently written to the magnetic tape medium prior to dismounting of the magnetic tape medium.
The controller 23 analyzes the commands and parameters stored in the buffer 22 to determine whether repositioning (driving) of magnetic tape is necessary. The controller tracks a current tape position, and determines direction and distance from the current position to a target position. A tape driver notifies the controller when the tape has been positioned to the target position. The controller 23 conducts, upon recognition of completion of tape positioning, data read and write operations via the magnetic head 27 depending on the command received from the host apparatus.
Importantly, conventional magnetic tape apparatus processes data access requests in the order in which they are received, i.e., without regard for the storage location of the data on the magnetic medium. As a result, data access throughput is sacrificed by the need to reposition the tape medium between each data access request.
A second problem associated with conventional magnetic tape apparatus relates to difficulty in allocating additional storage space for a given data item. A conventional tape apparatus stores each discrete data item in a continuous manner on the tape medium. Importantly, increasing the size of a data item requires the allocation of additional continuous storage space. If additional continuous space sufficient to accommodate the increase is not available, then the entire data item must be rewritten in another location in which sufficient continuous storage space is available. Obviously, this rewriting process is inefficient, and reduces data access throughput.
Accordingly, one object of the present invention to provide an improved tape apparatus which increases the throughput of data access requests by rearranging the sequence in which the requests are executed.
Moreover, another object of the present invention is to provide an improved tape apparatus capable of storing a discrete data item in a plurality of non-continuous regions on a tape medium, such that allocation of additional storage for a data item may be accomplished easily and efficiently.
The new and improved tape apparatus of the present invention includes a memory for at least one command queue table for storing a plurality of instructions from a host apparatus, and an execution queue table. The apparatus of the present invention further includes a reordering feature in which data access requests stored in the execution queue table are analyzed in conjunction with volume management information according to a predetermined set of rules. Importantly, the reordering feature changes the sequence of the data access requests stored in the execution queue table in order to minimize repositioning of the tape medium.
According to another embodiment of the invention, the tape apparatus has a control mechanism for recognizing a data storing position corresponding to a data access command using volume management information. The volume management information includes information for treating a plurality of non-continuous regions as a single data item. Thus, a discrete data item need not be stored in a continuous region of a tape. Moreover, the size of the discrete data item may be increased without the need for additional continuous storage space, and without the need to recopy the data item into a continuous region of the tape medium.
According to one aspect of the invention, the tape apparatus requests defragmentation processing when the number of non-continuous regions for a given data item increases beyond a predefined level.